Whilst MRI apparatus is always used for providing an image of the relevant part of the patient's body there are situations where in addition to merely providing the image some interventional action is taken with respect to the patient. A typical action would be to obtain a biopsy using a biopsy needle.
The present invention is concerned with such interventional activity and more particularly with the problem that the tools or devices used for such activity can and often do cause problems as far as the imaging is concerned.
It is known that unless such a device has virtually the same magnetic susceptibility as the tissue of the patient which surrounds that device it will give rise to a distorted image of itself and of the patient's surrounding tissue. Ideally the tool or device should have a magnetic susceptibility that does not deviate from that of the patient's tissue by more than several percent. A further practical problem is that the magnetic susceptibility of a patient's tissue is not fixed but varies depending upon the part of the body and in particular the blood flow or content.
Generally in the past, such devices or tools have been made of incompatible materials (i.e. badly matched materials), such as special stainless steels thus creating the problem that artifacts are generated which range from the total obliteration of the image to quite substantial increases in size and apparent changes of shape of the item being imaged.
Regulatory authorities have been concerned to define the term "MR compatibility" bearing in mind that achieving a perfect match may not be possible. Indeed, some workers have deliberately introduced susceptibility differences into small objects with too good a susceptibility match, so that they are very difficult to see on the images.
More specifically one problem is a mismatch between the apparent shape and size of the tool or device as displayed and its real shape and size. The extent of the distortion depends on the size, shape, the material and the orientation of the tool or device in relation to the main magnetic field.
The form of the artifact is complicated in interventional MRI by the diversity of equipment that is likely to be used in it and also by the fact that tools or devices can change their shape or configuration. For example, a pair of scissors can open or shut. A cannula which is being used to press a catheter or some other object into the body may initially be a hollow cylinder filled with, for example, blood, and may subsequently contain a guide wire or other device of a different susceptibility. This alteration could grossly change the apparent image appearance of the cannula.